Usually, for preventing the electronic device from being influenced by power damage or outage so as to operate stably, a backup power system is used. The backup power system is constituted by more than one independent power supplies and a control circuit connected thereto in parallel for switching or distributing power so as to provide power needed by the load. An example is R.O.C. application serial No. 200625865, entitled “Device circuit equipped with backup power”, wherein a backup power 805 is included. The backup power 805 includes two power sources 810, wherein only one of the two power sources 810 is used to supply power to the device, and the other power source 810 is used as a backup. This is a typical “1+1” redundant backup power system. In the “1+1” backup power system, multiple power sources are used to supply one operating system, and the backup power source is left unused when the operation is under normal operation without providing power thereto, so that the power resource of the backup power source is wasted. Furthermore, in the conventional backup power system, a protection circuit is also used to prevent the backup power system and the load connected thereto from being damaged by abnormal power, as disclosed in R.O.C. Patent No. I256758, entitled “Over-voltage protection circuit for power supply”. As shown in FIG. 2A and FIG. 3A of this patent, the backup power system includes multiple power supplies and each power supply is equipped with an over-voltage protection circuit, so that when over voltage occurs, the power supplies will be closed for avoiding damage. However, if the backup power system drives multiple loads and only one thereof occurs over voltage, the over-voltage protection circuit will close all the power supplies, that is, other normal loads will be forced to close even though only one load is abnormal. Therefore, there is a need to improve the conventional backup power system and the protection circuit.